Conventional boiling water reactors include auxiliary systems including, for example, a reactor water cleanup (RWCU) system, and a residual heat removal (RHR) system which is effective for several functions including shutdown cooling (SDC) of the reactor.
The RWCU system continually cleans the reactor water to remove impurities including fission and corrosion products, and is sized to process the water volume of the reactor system within a cycle that repeats typically every several hours. The system is typically operated during startup, shutdown, and refueling operation, as well as during normal plant operations. The system typically includes in serial flow communication with the reactor pressure vessel a pump for removing reactor water at reactor temperature and pressure (e.g. about 250.degree. C. and about 70 kg/cm.sup.2), regenerative and non-regenerative heat exchangers which cool the extracted reactor water, and a cleaner or filter-demineralizer, for removing the impurities. The cleaned flow is returned through the shell side of the regenerative heat exchanger where it is heated before being returned to the pressure vessel. The purified liquid effluent stream leaving the cleaner is also connected to a conventional main condenser and radiation waste tank, the former being used for also obtaining reactor blowdown in which excess reactor water is channeled through the RWCU system to the main condenser, and in the latter waste impurities are periodically purged thereto,
The RHR system typically includes for performing the shutdown cooling function a pump disposed in flow communication with the pressure vessel for pumping a portion of the reactor water to a non-regenerative heat exchanger wherein it is cooled and then returned to the pressure vessel through a conventional feedwater line or directly to the reactor pressure vessel. The RHR system is typically operated only at reactor pressures below about 9 kg/cm.sup.2, and is, therefore, designed for accommodating such relatively low pressure. Furthermore, the RHR shutdown cooling system is also sized for channeling reactor water from the pressure vessel at flowrates substantially larger than those used in the RWCU system in order to effectively cool the reactor water upon shutdown.
In each of the RWCU and RHR shutdown cooling systems described above, many of the components thereof are provided in duplicate to ensure redundancy of operation. For example, two each of the non-regenerative and regenerative heat exchangers and the cleaners are typically provided with crosstie pipelines being used for connecting them in parallel flow communication. Furthermore, numerous valves are also used for selectively blocking, switching, and regulating flow in the systems. The redundancy of components not only increases the reliability of the overall systems, it also allows each of the redundant components to be separately maintained while the reactor remains online and operational. In order to maintain or repair one of the redundant components, suitable valves are positioned for allowing operation of one of the components while isolating the redundant component from the flow circuit. The redundant component may then be removed or repaired while the overall system remains in operation.
However, the resulting systems are, therefore, relatively complex and include a substantial number of redundant components and valves. Maintenance requirements are. therefore, relatively high, and, many of the components may only be maintained or serviced upon shutdown of the reactor. For example, although the redundant heat exchangers are disposed in parallel flow with each other, they are typically disposed in serial flow in a single flow branch of the respective systems. Although each of the redundant heat exchangers may be serviced by closing off its respective valves to isolate the heat exchanger, those valves themselves may not be serviced unless the flow thereto is cut off which typically requires reactor shutdown.